1. Field of the Invention
The present invention relates to an optical semiconductor device, a semiconductor laser module, and an optical fiber amplifier.
2. Description of the Related Art
In recent years, increase in data traffic due to rapid spread of Internet or rapid increase in connections among intra-company LANs, etc. is a problem. Then, in order to solve that problem, a wavelength division multiplexing (WDM) system has achieved a remarkable development and become popular. In the WDM system, by multiplexing a plurality of signals onto lights each having different wavelength, large capacity transmission which is equal to or greater than 100 times relative to conventional cases is realized by using an optical fiber. In particular, in the WDM system, optical amplification by an optical fiber amplifier such as an erbium-doped optical fiber amplifier (hereinafter, EDFA) or a Raman amplifier etc. is indispensable, this optical amplification enables wide-band and long-haul transmission. Herein an EDFA is an optical fiber amplifier to which the principles applied that, when inputting pumping light having a wavelength of 1480 nm or a wavelength of 980 nm etc. from a pumping laser into a special optical fiber (hereinafter EDF) which is doped with erbium which is rare earth, 1550 nm band wavelength of light which is simultaneously-inputted transmission signal light is amplified in the above-described EDF.
Also, as a form of using EDFA, a so-called remote-pump-type is proposed, in which, when amplifying signal light in the middle of a transmission optical fiber laid on seabed, a pumping laser is disposed on land and pumping light outputted from the pumping laser is made input into the EDF through the transmission optical fiber. In the remote-pump-type EDFA, the pumping laser can be maintained or replaced easily by disposing the pumping laser on land.
On the other hand, a Raman amplifier is an optical fiber amplifier of a distributed type which, unlike an EDFA, does not need a special optical fiber such as an erbium-doped optical fiber but uses an optical fiber of an ordinary transmission path as a gain medium. Since the Raman amplifier has flat gain in wide-band, as compared with a WDM transmission system based on a conventional EDFA, the Raman amplifier has a feature capable of realizing wide-band transmission band. It should be noted that, in the Raman amplifier, since its amplification gain is lower than that of the EDFA, its pumping laser is required to have high output characteristics equal to or greater than that of the EDFA.
Therefore, in order to realize improvement in stability of the WDM system or reduction of the number of repeating, the pumping laser is required to have stable and high optical output capability. As the pumping laser, semiconductor laser devices having various structures such as a buried hetero (BH) structure etc. are used, and currently, for the above-described reason, development for especially a high power output semiconductor laser device is ongoing actively (see Jan P. van der Ziel, et. al., “InGaAsP (λ=1.3 μm) Stripe Buried Heterostructure Lasers Grown by MOCVD,” IEEE JOURNAL OF QUANTUM ELECTRONICS VOL. 27, NO. 11, pp. 2378-2385, 1991, Japanese Patent Application Laid-open No. 2000-174394, and Japanese Patent No. 3525257, which are referred as a Non-Patent document 1, Patent document 1 and Patent document 2, respectively).